3.2.2 Group 2, the alkaline earth metals

Question 1

How many electrons do group 2 elements lose?

Where from?

Answer 1

Group 2 metals lose their two outer electrons (from s²), to form 2+ ions, which have the electronic configuration of a noble gas.

Question 2

What is the trend in atomic radii descending Group 2?

Why?

Answer 2

Atomic radii increases descending the group. This is due to extra electron shells being added descending the group.

Question 3

What’s the trend in first ionisation energy descending Group 2?

Why?

Answer 3

First ionisation energy decreases down the group.

This is due to:
Each element down Group 2 having an extra electron shell compared to the one above
1. The inner shells shield the outer electrons form attration to the nucleus
2. Electron shells are also further away from the nucleus which also greatly reduces attraction to the nucleus

Due to weaker attraction to the nucleus, outer electrons are more easily removed

Question 4

What’s the trend in reactivity descending Group 2?

Why?

Answer 4

Reactivity increases down the group.

This is beacuse first ionisation energy decreases down the Group. As it’s easier to for the Group 2 element to lose electrons, the element is more reactive

Question 5

What is the trend in melting points descending Group 2?

Why?

Answer 5

Melting point generally decrease down the group.

1. Group 2 elements have metallic structures, with positive ions and a crystalline structure surrounded by delocalised electrons
2. Descending the group, metal ions get bigger. However, the no. of delocalised electrons doesn’t change or the charge on the ion
3. Due to a larger atomic radius down the group, the delocalised electrons are further away from the positive nuclei, therefore is less attraction.
   Therefore less energry is required to overcome overcome the metallic bonds, so melting point decreases.

Question 6

What is the excepetion to the trend in melting points of group 2 and why?

Answer 6

Magnesium, the crystal structure of the metal ions changes, causing it to be lower than expected.

Question 7

What happens when Group 2 metals react with water?

Answer 7

They are oxidised from a state of 0 to +2, forming Mg²⁺ions
E.g. M -> M²⁺ + 2e^-

Group 2 metals react with water to give a metal hydroxide and hydrogen
E.g. M_(s) + 2H₂O_(l) -> M(OH)_2(aq) + H_2(g)

Question 8

How does solubilty of Group 2 compounds vary depending on the charge of the compound anion?

Answer 8

Generally, compounds of Group 2 elements that contain a singly charged negative ions (e.g. OH^-) increase in solubility down the group, whereas compounds that contain doubly charged negative ions (e.g. SO₄^2-) decrease in solubility down the group.

Question 9

What is the test for sulfate ions?

Answer 9

Barium sulfate (BaSO₄) is insoluble in water, therefore when Barium Chloride (BaCl₂) is added to a solution containing Sulfate ions a white precipitate is formed.

The solution needs to be acidified using hydrochoric acid to remove and sulfites or carbonates which can also produce a white precipitate

Question 10

Name two uses of compounds of Group 2 elements

Hint: alkaline earth metals

Answer 10

1. Calcium hydroxide (Ca(OH)₂) is used in agriculture to neutralise acidic soil
2. Magnesium hydroxide (Mg(OH)₂) is used in some indigestion tablets as an antacid to neutralize excess stomach acid

Question 11

Explain how barium sulfate can be used in X-rays

Answer 11

Barium sulfate is opaque to X-rays, therefore “Barium meals” can be eaten to to diagnose problems with the oesophagus, stomach or intestines.
A patient swallows a barium meal, which is a suspension of barium sulfate, the barium sulfate coats the tissues, making them show up on X-rays, showing the structure of organs.

Question 12

How is Magnesium used in the extraction of titanium?

Answer 12

The main titanium ore, titanium(IV) oxide (TiO₂) is first converted to titanium (IV) chloride (TiCl₄) by heating it with carbon in a stream of chlorine gas.
The titanium chloride is then purified by fractional distillation, before being reduced by magnesium in a furnace at almost 1000^oC

TiCl_4(g) + 2Mg_(l) -> Ti_(s) + 2MgCl_2(l)

Question 13

How are Calcium Oxide and Calcium Carbonate used to remove Sulfer Dioxide produced from burning fossil fuels?

What are the equations for this?

Answer 13

1. The acidic sulfer dioxide can be removed from flue gases by reacting with an alkali in a process called wet scrubbing
2. Powdered calcium oxide (lime,CaO) and calcium carbonate (limestone, CaCO₃) can both be used for this
3. A slurry is made by mixing the calcium oxide/carbonate with water
4. The slurry is then sprayed onto the flue gases.
5. The sulfer dioxide reacts with the alkaline slurry and produces a solid waste product, calcium sulfite

CaO_(s) + 2H₂O_(l) + SO_2(g) -> CaSO_3(s) + 2H₂O_(l)

CaCO_3(s) + 2H₂O_(l) + SO_2(g) -> CaSO_3(s) + 2H₂O_(l) + CO₂